Ahead of Mother’s Day, a new report offers a geographic and historical look at working motherhood in the U.S.

Among the findings: South Dakota is the state with the highest percentage of working mothers in the United States, according to a new analysis of 150 years of U.S. Census data compiled by the online history database Ancestry.com.

The Mount Rushmore state has topped the nation in working mothers per capita for the past three decades, according to Ancestry.com, which collaborated with a team of historians at the University of Minnesota to crunch the numbers going back to 1860.

Michelle Ercanbrack, a historian who works for Ancestry.com is careful to note that “any mother is a working mother,” but the purposes of this survey, working mothers are defined as women who reported having an occupation to the Census bureau and who lived at the time with at least one child in their household.

The report highlights dramatic shifts in the landscape of the American workforce. In 1860, according to the data, just 7.5% of the nation’s mothers worked outside of the house. By 2010, that figure stood at 67%.

The proportion of working mothers grew by double-digit percentages each decade from 1950 to 1990, a spike initially attributed to World War II, which pulled millions of women into the workforce for the first time, and sustained in part by the Civil Rights movement, overall economic prosperity and the rise of single mothers.

In 2013, there were some 10 million single mothers in the U.S., up from 3.4 million in 1970, according to Census figures.

As of the latest Census, 79.9% of mothers in South Dakota worked, and the state is one of several Midwestern states in the top 10. That’s a sharp reversal from 1930, when the state ranked at the very bottom of the list.

Ercanbrack speculates that because the region’s economy was largely agricultural a century ago, Midwestern women were unlikely to report having jobs outside of helping to run the farm. Today, South Dakota has one of the nation’s lowest unemployment rates, with large service and tourism industries, and its neighbor to the north is enjoying a booming oil industry.

Meanwhile, southern states such as South Carolina and Alabama, which reported some of the highest rates of working mothers in 1930, now populate the bottom of the list. Ercanbrack says the earlier rates of working mothers in the region were inflated by the lingering effects of Reconstruction; today’s relatively low percentages may be a symptom of high unemployment generally.

Todd Goffrey, who acquires historical records for Ancestry.com, says he hopes the data will spur more researchers to look into the shifts in motherhood and employment, and working mothers’ influence on the American labor market. But he also views the statistics as a window into the lives of individual ancestors, many of whom left behind few records aside from their answers to Census questions.

“Think about our typical member, who sees Grandma [in a Census report] and is trying to understand why she was the way she was,” he says. “These trends are a piece of the puzzle.”

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